In its most common form silver halide photography employs silver halide grains that are precipitated in the presence of an aqueous gelatino-peptizer. After precipitation the grains are chemically sensitized and, if desired, spectrally sensitized. The emulsion is coated on a support to form a photographic element. Following imagewise exposure the photographic element is brought into contact with one or more aqueous processing solutions. During aqueous processing a developing agent is relied on to reduce silver halide to silver imagewise as a function of light exposure and, in color photography, to create a dye image.
Silver halide photographic elements and their chemical sensitization are summarized in Research Disclosure, Vol. 389, September 1996, Item 38957. Common chemical sensitizations are summarized in Section IV. Chemical sensitizers are generally assigned to three broad categories: (1) noble metal sensitizers, including gold and Group VIII metals, (2) middle chalcogen sensitizers, including sulfur, selenium and tellurium sensitizers, and (3) reduction sensitizers.
The following are specific examples of gold sensitizers: Tetrachloroaurate salts, disclosed by Damschroder U.S. Pat. No. 2,597,856 and Hasebe U.S. Pat. No. 5,001,042; dithiocyanatoaurate salts, disclosed by Saikawa et al U.S. Pat. No. 4,621,041; gold sulfide, disclosed by Yutzy et al U.S. Pat. No. 2,597,915, Yutzy et al U.S. Pat. No. 2,642,361, and Masutomi et al U.S. Pat. No. 5,362,470; gold thiolates, disclosed by Travernier et al U.S. Pat. No. 3,503,749; gold compounds that satisfy the formula: EQU AuL.sub.2.sup.+ X.sup.- or AuL(L').sup.+ L.sup.-
wherein
L is a mesoionic ligand; PA1 L' is a Lewis basic donor ligand; and PA1 X is an anion, PA1 M represents a charge balancing cation, PA1 (a) photosensitive silver halide grains, PA1 (b) an oxidation-reduction image-forming combination comprised of PA1 (c) a vehicle. PA1 I. System A photothermographic systems, aqueous silver halide precipitations and vehicle: PA1 II. System B photothermographic systems, aqueous silver halide precipitations: PA1 III. System B photothermographic systems, in situ silver halide precipitation: PA1 IV. System B photothermographic systems, ex situ silver halide precipitation in non-aqueous polymeric peptizer: PA1 X is PR.sub.2, P(OR).sub.2, COR, CNR.sub.2 or CR; PA1 R is alkyl or aryl; and PA1 n is an integer of from 1 to 6. PA1 I. Tetrachloroaurate salts, such as those satisfying the formula M{AuCl.sub.4 } (M=H, Na, K). Gold sensitizers of this type are disclosed by Damschroder U.S. Pat. No. 2,597,856 and Hasebe U.S. Pat. No. 5,001,042, the disclosures of which are here incorporated by reference. PA1 II. Dithiocyanatoaurate salts, such as those satisfying the formula M{Au(SCN).sub.2 } (M=K, NH.sub.4). Gold sensitizers of this type are disclosed by Saikawa et al U.S. Pat. No. 4,621,041 and Hasebe, U.S. Pat. No. 5,001,042, the disclosures of which are here incorporated by reference. PA1 III. Gold dithiosulfate salts, such as those satisfying the formula Na.sub.3 {Au(S.sub.2 O.sub.3).sub.2 }.2H.sub.2 O. Gold sensitizers of this type are disclosed by P. Faelens, R. Berendser, and B. H. Tavernier, Phot Korr., 53, 75 (1966). PA1 IV. Gold sulfide (Au.sub.2 S), disclosed by Yutzy et al U.S. Pat. No. 2,597,915, Damschroder et al U.S. Pat. No. 2,642,361, and Masutomi et al U.S. Pat. No. 5,362,470, the disclosures of which are here incorporated by reference. PA1 V. Gold thiolates, such as those satisfying the formula AuS--X--SO.sub.3 M, wherein X is an aliphatic aromatic or heterocyclic bivalent radical, and M is H, an alkali or alkaline earth metal, or NH.sub.4. Gold sensitizers of this type are disclosed by Travernier et al U.S. Pat. No. 3,503,749. PA1 VI. Gold compounds of the formula {AuL.sub.2 }.sup.+ X.sup.- or {AuLL'}.sup.+ X.sup.- wherein L is a mesoionic ligand, L' is a Lewis base donor ligand, and X is an anion (e.g.,BF.sub.4.sup.-). The mesionic ligand is typically a cyclic or acyclic thiourea. Gold sensitizers of this type are disclosed by Deaton U.S. Pat. Nos. 5,049,485 and 5,220,030, the disclosure of which is here incorporated by reference. Gold compounds of this type are particularly preferred, since higher levels of photosensitivity have been realized by employing these compounds in the practice of the invention than when employing other types of gold compounds as sensitizers. PA1 R.sub.1 and R.sub.4 are independently chosen lower alkyl (e.g., methyl); PA1 R.sub.2, R.sub.3 and R.sub.5 are independently chosen from among lower alkyl (e.g., methyl), alkoxyalkyl (e.g., .beta.-methoxyethyl), allyl, aryl (e.g., phenyl), or cycloalkyl (e.g., cyclohexyl); and PA1 X.sup.- is an anion chosen from among BF.sub.4.sup.-, I.sup.-, Br.sup.- and Cl.sup.-. PA1 VII. Development Modifiers PA1 X. Coating Aids PA1 XI. Overcoat Layers PA1 XII. Interlayer/Barrier Layers PA1 XIV. Filter Dyes/Antihalation Layers PA1 XV. Color Materials PA1 XVII. Supports
disclosed by Deaton U.S. Pat. Nos. 5,049,485 and 5,252,455; and gold compounds satisfying the formula: ##STR1## wherein Z represents the atoms to complete a 5 or 6 membered heterocyclic ring and
disclosed by Deaton U.S. Pat. No. 5,049,484.
The advantage of photographic imaging as described above is that the highest attainable levels of photosensitivity are realized. A significant disadvantage is the requirement of bringing the photographic elements into contact with an aqueous processing solution during development--i.e., "wet" processing.
A way of avoiding wet processing is to employ a photosensitive element that can create a viewable "dry" image simply by being heated following imagewise exposure. Research Disclosure, Vol 170, June 1978, Item 17029, provides a summary of the many varied common photothermographic imaging systems. These photothermographic imaging systems have been broadly divided into Systems A and B.
In System A the photothermographic composition is comprised of photosensitive silver halide grains that are relied upon as the sole source of silver. System A has the advantage of being able to employ aqueous silver halide emulsions of the type employed in silver halide photographic systems, with their superior photosensitivity.
Photothermographic imaging systems predominantly fall into the System B category. The minimum essential components of System B are the following:
(i) an organic silver compound and PA2 (ii) an organic reducing agent and PA2 Sakaguchi U.S. Pat. No. 4,639,414. PA2 Lindholm et al U.S. Pat. No. 3,700,458; PA2 Ikenoue et al U.S. Pat. No. 3,909,271; PA2 Reeves et al U.S. Pat. No. 4,435,499; and PA2 Zou et al U.S. Pat. No. 5,434,043. PA2 Ohkubo et al U.S. Pat. No. 3,887,597 and PA2 Naito et al U.S. Pat. No. 4,478,927. PA2 Lindholm et al U.S. Pat. No. 3,713,833; PA2 Jones U.S. Pat. No. 3,871,887; PA2 Hiller U.S. Pat. No. 4,207,108; PA2 Kagami et al U.S. Pat. No. 4,725,534; and PA2 Pham U.S. Pat. No. 4,751,176. PA2 System B. PA2 A. Paper Supports PA2 B. Film Supports
The organic silver compounds that are most efficiently catalyzed by light-exposed silver halide grains to enter into an oxidation-reduction reaction in response to heating are those that are most conveniently dispersed in non-aqueous polymeric vehicles. Thus, although techniques are known for combining aqueous silver halide emulsions with the oxidation-reduction combination, the disadvantages of combining silver halide grains precipitated in an aqueous peptizer with a non-aqueous polymeric vehicle containing the oxidation-reduction combination more than offset any advantage of this approach. Therefore, it is generally preferred and most convenient to form the silver halide grains of component (a) in the presence of a non-aqueous polymeric peptizer.
Silver halide grains are formed in the presence of non-aqueous polymeric peptizer using either in situ or ex situ techniques. The in situ technique converts a portion of the organic silver compound (i) of the oxidation-reduction combination (b) to silver halide. The disadvantage of this technique is that chemical sensitization of the silver halide to increase photosensitivity is difficult. Typically chemical sensitization involves (1) adding a chemical sensitizing compound and (2) holding the silver halide at an elevated temperature for a period sufficient to allow interaction of the silver halide and the sensitizer. Since the in situ formation of the silver halide requires the presence of the organic silver compound, which is necessarily heat sensitive, it is apparent that heating runs the risk of unwanted silver ion release, thereby degrading image quality.
This disadvantage is avoided by undertaking ex situ silver halide grain formation, wherein, by definition, the organic silver compound is not present while the silver halide grains are being prepared. In addition, silver halide grain formation in the absence of the organic silver compound is generally simpler and more convenient than in situ grain formation.
In most instances photothermographic compositions are prepared and used without employing any chemical sensitization step comparable to that employed in preparing silver halide photographic emulsions. This is because silver halide photothermographic compositions are generally much slower than silver halide photographic compositions and have been commonly relegated to imaging applications that are compatible with low imaging speeds.
Research Disclosurei Item 17029, cited above, Section VI. Chemical Sensitization/Speed Addenda discloses a variety of sensitizations that have been developed for chemically sensitizing silver halide emulsions containing non-aqueous polymeric peptizers and silver halide formed in the non-aqueous polymeric vehicles of System B.
The following patents particularly directed to photothermography have been considered:
Lushington and Gysling U.S. Pat. No. 5,759,761, filed Jun. 4, 1997, titled GOLD CHEMICAL SENSITIZERS FOR SILVER HALIDES, commonly assigned, is directed to silver halide grains sensitized with at least one gold compound of the formula: EQU {AuS.sub.2 X}.sub.n
wherein